1. Field of the Invention
This invention relates generally to signals and alarms and more specifically to the combination within a single module and in a compatible manner of both a piezoelectric audible signal and a light emitting visual signal.
2. Description of the Related Art
The prior art has long known a diverse variety of signals that are commonly used to signal alarm conditions and other events. These include audible signals as well as visual signals that commonly emit light sometimes according to a timed sequence pattern of illumination.
One of the best performing and most cost effective audible signals is the piezoelectric alarm that has a diaphragm driven by an electrically excited piezoelectric crystal. Commonly, its components are all housed within a case that includes a tubular wall and an exposed cover at one end. Typically, the diaphragm is mounted within the tubular wall and, when the signal is operably installed, the tubular wall extends through a hole in a panel, such as a control panel. However, the “panel” may also be a mounting bracket or other support structure. The exposed cover ordinarily seats against the front face of the panel in a position where it is exposed to the view of an operator. A sound enhancing cavity is formed within the tubular case and extends from the diaphragm to the exposed cover. The cover has one or more apertures for emitting from the cavity the sound generated by the vibrating diaphragm. The cavity is ordinarily a Helmholtz cavity which is a resonant cavity with one or more apertures for the escape of sound waves from the cavity. The cavity is dimensioned so that it is resonant in order to impedance match the diaphragm to a volume of air in the cavity so that the maximum audio power is coupled from the diaphragm to the air within the cavity. That ultimately provides the maximum intensity for the sound being emitted out of the case through the apertures.
The prior art has also shown an extensive variety of visual signals for indicating an alarm or other condition by illuminating a light source in a steady, flashing or other pattern. These visual signals are also commonly mounted to a panel and contain a light source that directly illuminates a transparent or translucent cover from which the light is transmitted to an observer.
In addition to the many separately used audible and visual signals, it is not uncommon to use both an audible signal and a separate visual signal to simultaneously signal an event, such as an alarm condition. This redundancy directs the signal to both the human sense of sight and the sense of hearing in order to increase the probability that the signal will come to the attention of a human operator or observer. An example is shown in U.S. Pat. No. 4,019,607.
The prior art has also shown signals which incorporate both an audible and a visual signal within the same signal unit. For many types of audible and visual signals, combining them in the same signal unit or case presents no major problems. However, significant difficulties arise when attempting to combine an illuminated visual signal with an audible piezoelectric signal in a manner that maintains high sound quality and intensity, durability and low cost for which piezoelectric signals are known. A major difficulty arises because the diaphragm is an opaque metal disk that prevents light sources from transmitting light through the diaphragm. Furthermore, there are at least the following four conditions that need to simultaneously coexist in a combined illuminated visual signal and an audible piezoelectric signal if the performance and cost expectations of customers, based upon their experience with previously marketed but separate audible and visual signals, is to be met.
(1) The first required condition is that the piezoelectric signal must have a sound enhancing cavity, most desirably a Helmholtz cavity, within its tubular wall between the diaphragm and the front cover with its sound emitting apertures.
(2) The second required condition is that there should be no object in the cavity because an object in the cavity would interfere with the resonance or otherwise deteriorate the intensity and/or the audio characteristics of the sound emitted from the cavity through the apertures in the cover. Consequently, the light sources themselves can not be mounted in the cavity where they could very effectively transmit light directly to the exposed cover but would interfere with the sound and cavity resonance. Furthermore, even the presence of wires extending through the cavity and through a hole in the diaphragm, as taught in U.S. Pat. Nos. 6,130,618 and 6,414,604, have a deteriorating effect upon the sound. The cavity has interior surfaces which reflect the wave. When a wave at the resonant frequency of the cavity is generated by the vibration of the diaphragm, the wave bounces back and forth within the cavity, with low loss. If the cavity were closed, a pure standing wave would be generated in the cavity. As more wave energy enters the cavity, it combines with and reinforces the standing wave, increasing its intensity. A Helmholtz cavity is a container with an aperture. The aperture allows sound waves to be emitted from the cavity but is smaller than the cavity so that some standing wave reinforcement is accomplished to improve the audible signal strength. In a prior art system that places structures in the cavity for the purpose of providing a visual signal, these structures cause additional wave reflections within the cavity resulting in deflection and some cancellation of the reflected waves in the cavity and they can also absorb some acoustic energy. Consequently, these internal structures deteriorate the resonant properties of the cavity and therefore reduce the strength and clarity of the output audible signal emitted from the cavity orifice.
(3) The third required condition is that, in order to have an effective cavity, there must be a significant spacing between the diaphragm and the exposed cover. If the signal has such a cavity that is devoid of any physical object and the exposed cover seats in the customary manner against the front surface of the panel to which the signal is mounted, then the diaphragm must be spaced behind the plane of the panel in order that the cavity have sufficient volume and wall spacing. With the diaphragm spaced behind the plane of the panel, the light source can not be placed radially inwardly from the periphery of the exposed cover or the light source would be within the cavity. Consequently, the solution in U.S. Pat. No. 4,904,982, which places the diaphragm in front of the plane of the panel and the light sources immediately behind the diaphragm, becomes impractical when there is a cavity. Of course it would also be possible to position the diaphragm far enough in front of the plane of the panel in order to have a cavity and place the light source immediately behind the diaphragm but within the exposed cover. But the result of doing so would be that the tubular wall and/or the exposed cover would protrude a distance out from the panel that would make the signal intrusive, more likely to be struck and broken by some other object, aesthetically unacceptable to customers and possibly dangerous.
(4) Mounted within the tubular wall of the case of an optimally designed signal are both the diaphragm and a printed circuit board that contains all the electrical circuitry for the signal. Not only must the diaphragm of a practical signal be spaced back from the front cover beyond the plane of the panel with nothing in the cavity, but also the circuit board must be on the opposite side of the diaphragm from the exposed panel. The fourth condition, that is highly desirable for maintaining product quality and low cost, is that the diaphragm and the printed circuit board need to extend radially outwardly to reach the inner surface of the tubular wall of the case so that both can be rigidly and inexpensively mechanically mounted to the case, such as with an adhesive or mechanical clamping. If the circular diaphragm did not extend to the tubular wall, but instead left a space for light to pass beside the diaphragm, additional small mounting parts would be needed, making the signal more expensive to fabricate and the diaphragm and the mounting parts would be more likely to come loose from the tubular wall as a result of vibration or sudden acceleration.
It is therefore an object of the present invention to integrate a visual signal, that emits light, into the same module that houses a piezoelectric audible signal without diminishing the space available for a cavity and without having structures within the cavity that can diminish its resonant characteristics and to do so with mechanically simple parts that can be easily and inexpensively assembled in a durable configuration.